Optical technologies and sciences related to such fields as microscopy have evolved from ancient observations and understandings of the nature of light to the manner in which light can be manipulated via one or more optical devices such as through a lens. In fact, some sources have cited that Zacharias Jansen—of Holland in 1595, was possibly the first inventor of a multiple lens or compound microscope design. After Jansen, many improvements were incorporated into microscope designs over the centuries leading up to Lord Rayleigh's and Ernst Abbe's discoveries in the 19th century regarding diffraction limitations in lenses. These scientists demonstrated that physical laws of diffraction require that a minimum resolving distance of a lens is related to the wavelength of light divided by a parameter referred to as the Numeric Aperture of the lens. By the 1880's, oil immersion objective lenses were developed having a Numeric Aperture of about 1.4—leading the way for light microscopes to resolve between two small points at about the theoretical diffraction limits established by Rayleigh and Abbe. The resolution demonstrated by lenses operating at the limits of diffraction theory, however, is rarely achieved in practice without sacrificing other desirable characteristics. For instance, as light microscope designs continued to develop in the 20th century, increased magnification of smaller and smaller objects also continued, whereby many of the best microscope designs can offer visually “pleasing images” at about 1000 times magnification. Unfortunately, increased magnification in conventional microscope designs generally causes tradeoffs in other design features such as resolution and contrast.
In order to illustrate these tradeoffs, the following discussion provides a conventional microscope design methodology that has developed over the ages. Conventional microscope designs limit useful magnifications to approximately 1000 times (×) since the intrinsic spatial resolution of the lenses cannot exceed limits dictated by the well-known Rayleigh equation:R=1.22λ/(NAOBJECT+NACONDENSER)
Thus, for a conventional 100× high resolution, “Infinity-Corrected”, oil immersion objective lens, having a standard maximum Numerical Aperture of 1.25, utilized in conjunction with a regularly employed setting for the highest contrast of a sub-stage, in-air lighting condenser, having a Numerical Aperture of 0.9 employed in conjunction with oil-immersion condensers having a Numerical Aperture of up to 1.4 (e.g., modern Kohler Lighting configurations), and applied at a standard illumination wavelength of 0.55 micron, for example, the resulting known best theoretical spatial resolution at the highest useful magnification is therefore about 0.312 microns (312 nanometers). Any increase in magnification increases image size but also results in well-known increased detail blur at the image plane. Consequently, typical best visual spatial resolution is based on contrast and magnification of so-called “pleasing images” and rarely actually exceeds 500 nanometers (0.5 microns) and is regularly on the order of 1000 nanometers (1 micron).
In modern times, optical designs have been applied to other technologies such as digital imaging, machine vision for direct imaging, inspection, fiducial and absolute measurement, counting, characterizing geometry, morphology, coordinate location, spectral information, analytical imaging for identification, medical clinical microscopic imaging, and a plurality of other image-based applications. In addition, video imaging techniques and associated computerized image processing methods have long been a standard inspection technique in many industries and applications. High resolution and high magnification video-based imaging systems have conventionally relied upon known techniques of conventional microscopic instrumentation coupled to a video camera or other device. Other variations have typically employed well-known “macro” and “tele-zoom” optical lens components, (long range and short range) coupled to video camera devices to achieve high magnification as well. Though many of the imaging applications mentioned above, employ these techniques regularly, the methods have been subject to optical and illumination related limitations that can cause substantial degradation of image quality and usefulness when high magnification and resolution are required.
Well-defined and known limitations of conventional high-magnification and/or high-resolution imaging systems include but are not limited to:
(1) Very narrow Field Of View (FOV) and very small Working Distance (WD) for high effective magnification;
(2) High Effective Magnification limited to “useful magnification” at accepted maximum of about 1000× and is determined by well-known optical diffraction effects which govern absolute possible spatial resolution in optical images;
(3) Very small Depth Of Field (DOF) typically less than 1 micron at high magnification; Inhomogeneous illumination sources (varying intensity across even a small field) are extremely position sensitive for correct magnification and contrast vs. spatial resolution for non-quantifiable “pleasing appearance” versus well known “empty resolution” in clinical and industrial microscopy;
(4) Objective lens to object distance decreases in operation from low to high power objective lenses in order to increase effective magnification (typical 15 to 20 mm for low power objective to fraction of a millimeter for up to 50× objectives;
(5) Highest Numerical Aperture is required for high magnification and is generally only achievable with immersion-type objective lenses; and
(6) Very high Effective Magnification generally requires 50× to 100× objective lenses typical for object image projection to magnifying eyepiece or subsequent imaging device and have inherently short working distance and very small Field Of View as well as other limitations, including “empty magnification” problems.
Other problems with conventional imaging systems relate to oil immersion objective lenses to increase the numerical aperture through Index of Refraction matching fluid such as oil for objective magnification over 50× are typically (e.g., at 100×) required to achieve effective through-the-eyepiece magnifications of up to 1000×. This also requires extremely small objective lens to object spacing through the oil medium of approximately 100 microns or less. Other issues involve the small “circle of least confusion” (object plane image diameter) magnified by an inspection lens system (generally an optical eyepiece or equivalent) for projection onto an image sensor limiting spatial resolution to a number of sensor pixels across a projected image on to the sensor. This inherently limits both a Modulation Transfer Function that defines contrast versus resolution and absolute spatial resolution.
Still other problems can involve conventional “Infinity-corrected” microscope objectives that are designed with optical parameter correction for an effective “infinite-tube-length”, thus these lenses can require a telescope objective lens (also called the “tube-lens”) in addition to an eyepiece to bring the image into focus for the eye. Such systems are known to permit a convenient modular, or building-block concept of design since fairly sizeable accessories can be inserted into the infinity space without upsetting tube length, magnification, parfocality, working distance, or axial image quality. Though microscope systems employing infinity-corrected objective lens designs are widely available, these systems are still designed via the conventional method of magnifying small objects in the field of view from the object plane through the “tube-lens” (telescope objective) to the eyepiece for viewing, or through a special magnifying lens to an imaging device (photographic or electronic). This is an accepted method of optical design employing geometrical optics design rules and results in even the most advanced conventional microscopic imaging systems having the aforementioned well-known limitations in projected Field Of View, Effective Magnification, Absolute Spatial Resolution, and Diffraction Limitations at the imaging device.
Generally, the design purpose of instruments employing conventional infinity-corrected microscope objectives is to permit the placement of certain auxiliary optical and illumination components in optical path length between the objective and image sensor. This region known as the “infinity space” is designed to introduce minimal aberrations and other unwanted optical effects. However, even the most advanced systems generally limit to two the number of such additional added components without specifying additional correcting optics.
Another problem with conventional high magnification image designs relates to special configurations to employ either transmissive or reflected illumination techniques. This can include special microscopic variations such as cardioid or paraboloid condensers, fluorescence and interference microscopy attachments, as well as typical machine vision illumination schemes (e.g., darkfield, brightfield, phase-contrast, and so forth), and conventional microscopic transmissive illumination techniques (Kohler, Abbe) that typically require vastly different optical imaging designs by nature and are generally mutually exclusive. These designs are also labor intensive for operational adjustment and for optimum image quality from sample to sample under examination. As can be appreciated, modern optical designs employing high-grade oil immersion lenses and/or other correcting optics generally involves significant expense.